1. Field of the Invention
This invention relates to coated, abrasion resistant press plates used in making abrasion resistant decorative laminate, to the coating of press plates and to the making of laminate with these press plates. Grit, e.g., alumina particles, on the pressing surface of abrasion resistant decorative laminate can scratch press plates and reduce the visual quality of laminate thereafter made with the press plate. Press plates of this invention are particularly useful in making abrasion resistant high gloss decorative laminate.
2. Description of the Related Art
In the manufacture of decorative laminate, layers of resin impregnated paper are pressed against press plates under conditions of temperature and pressure to cure the resin and bond the layers together. A high gloss press plate imparts a high gloss surface to laminate. A textured surface imparts a textured surface to laminate. These press plates are extremely uniform, with even microscopic discontinuities being minimized. The quality of a high gloss polished press plate can be determined by viewing reflected images on its surface and scrutinizing the reflected images for optical discrepancies. Grit on the surface of laminate causes micro scratching of stainless steel press plates normally used in the manufacture of decorative laminate, thus destroying the micro finish of the press plate. Press plates can also be scratched by press plate handling equipment and by debris from pressing equipment or materials used in making laminate. (Laurence U.S. Pat. No. 5,244,375)
Melamine resin coated decorative laminate is pressed at temperatures of about 230-310.degree. F. (110-155.degree. C.) and pressures of about 300-2000 psi (20-136 bar) and preferably about 750-1500 psi (51-102 bar). Heating to these temperatures and cooling to room temperature results in substantial expansion and contraction of the laminate and of the press plate. Expansion and contraction of the laminate and press plate will not be the same, resulting in the movement of grit on the pressing surface of laminate across the press plate.
It is disclosed in National Electrical Manufacturers Association (NEMA) Standards Publication No. LD 3, that gloss finish laminate has a gloss of 70-100+. High gloss textured finish laminate is disclosed as having a gloss of 21-40. Black glass with a gloss of 94.+-.1 degrees, measured at an angle of 60 degrees, is disclosed as the NEMA Standard 3.13.2, for calibrating a gloss meter for 60 degree angle gloss measurements.
Even discontinuities in high gloss press plates that can only be seen with a microscope can impart visible surface defects to a high gloss surface of laminate. Any scratching of high gloss press plates imparts visible surface defects to high gloss surfaces of laminate and reduce gloss level.
Grit on the decorative surface of laminate imparts abrasion resistance, a commercially desirable characteristic of laminate. Particles of alumina are commonly used as grit in making decorative laminate. The Vickers hardness of alumina is disclosed in "Tribology: Friction and wear of Engineering Materials", I. M. Hutchings, CRC Press, 1992, to be 1800 to 2000. A useful range of particle sizes is about 10 to about 75 microns. Grit of about 25-60 microns is preferred. Optimum abrasion resistance is obtained in the particle size range of about 40 to 60 microns. (Lane et. al. U.S. Pat. No. 3,798,111)
Alumina having a maximum particle size of 9 microns is disclosed as being effective for imparting a wear resistant surface to glossy decorative laminate. Wear resistance is defined as the resistance of a glossy laminate to loss of gloss when the surface of laminate is exposed to the abrasive effects of sliding objects. It is acknowledged that the resulting laminate does not meet NEMA LD 3.01 requirements to be considered as abrasion resistant. However, it is disclosed that glossy press plates are not scratched substantially if the grit particle size is maintained at less than 9 microns. (Lex et. al. U.S. Pat. No. 4,971,855)
The use of a 410 stainless steel press plate hardened by nitriding is disclosed for making high gloss decorative laminate. After pressing 100 sheets of high gloss laminate with 6 micron and 15 micron grit, the gloss of the pressed laminate remained good to very good. The nitrided press plate exposed to the 6 micron grit was rebuffed after 234 cycles and produced acceptable laminate quality for at least another 103 cycles. Nitrided press plates exposed to 30 micron grit offered limited durability. It is disclosed that the 410 stainless steel press plate used for nitriding had a Rockwell, "C" scale hardness of 38-45 and that the nitrided surface had a Rockwell, "C" scale hardness of 60-70. The equivalent Vickers hardness of 410 stainless steel is about 370-440, based on a conversion table published in "Metals Handbook, Mechanical Testing", Vol. 8, 9th ed., ASM, 1985. The equivalent Vickers hardness of nitrided 410 stainless steel is about 500-1000, based on a conversion table published in "Metals Handbook, Mechanical Testing", Vol. 8, 9th ed., ASM, 1985. (Laurence U.S. Pat. No. 5,244,375)
Laminate with 35 micron average particle size alumina at its surface (PGA 822 overlay, available commercially from Mead Corporation) has been pressed with high gloss press plates coated with titanium nitride. After ten pressings, the titanium nitride coated press plates had about 15 scratches per square centimeter. A control 410 stainless steel press plate had about 500 scratches per square centimeter. The Vickers hardness of titanium nitride is disclosed in "Tribology: Friction and wear of Engineering Materials", I. M. Hutchings, CRC Press, 1992, to be 1200 to 2000.
The control press plate and the press plate on which the titanium nitride was coated were cut from the same stainless steel pressing plate. The scratches was visible under a light microscope at 40X magnification. Titanium nitride was coated onto 410 stainless steel high gloss press plates in a magnetron sputter coating system. The use of a magnetron sputter coating system for applying a titanium nitride coating is disclosed in "Multi-Cathode Unbalanced Magnetron Sputtering Systems," Sproul, Surface and coating Technology, 49 (1991). The use of a magnetron sputter coating system for cleaning the surface that is to be coated is disclosed in "A New Sputter Cleaning System For Metallic Substrates," Schiller et. al., Thin Solid Films, 33 (1976).
Additionally, the color of the laminate pressed with the titanium nitride coated press plate was different than the color of the laminate pressed with the control press plate. An ASTM D 2244 color difference in comparison to a standard of less than (.+-.0.5) .DELTA.E is considered as an acceptable color match to the standard. The ASTM D 2244 color difference between a standard and laminate pressed with the titanium nitride coated press plate was greater than (0.5) .DELTA.E. The titanium nitride coated press plate and laminate pressed therefrom had a bronze appearance. The control press plate and the laminate pressed therefrom did not have a bronze appearance. Laminate pressed with the control press plate had an ASTM D 2244 color difference when compared with the standard of less than (0.5) .DELTA.E.
Iron-based cutting tools have been sputter coated with 2-6 microns of titanium diboride. The sputtering is carried out in an argon or krypton beam of ions accelerated to 1300-1800 volts as a broad-beam ion source. A titanium diboride target is arranged as a cathode. The tool is heated to about 200.degree. C. (392.degree. F.). Sputtering is done under a vacuum of about 4-6 milli-Torr. Titanium diboride has an extremely high Vickers micro-hardness value, typically about 3600, which is not only considerably higher than other borides but also substantially higher than other carbides or nitrides. Titanium diboride is also particularly noted for its high density, e.g., 88% of theoretical density, a low resistivity of 30 micro-ohms centimeters, a high strength of about 40,000 psi, and a coefficient of thermal expansion which is about 8.1.times.10.sup.-6 at the temperature range of 20.degree.-800.degree. C. (68-1472.degree. F.). (Moskowitz et al., U.S. Pat. No. 4,820,392)
Control conditions for sputter coating are disclosed in "Influence of Apparatus Geometry and Deposition Conditions on the Structure and Topography of Thick Sputtered Coatings" Thornton, Journal of Vacuum Science Technology, Volume 11, Number 4, (July/August 1974) and "Sputtering" Thornton et al., Metals Handbook, Ninth Edition, American Society for Metals, Metals Park, Ohio, 44073, Volume 5, pp 412-416, (1982).
There is a need for a hard coating on a press plates, continuous belt, and other pressing surfaces that imparts a color to laminate having an ASTM D 2244 color difference in comparison to a standard of less than (.+-.0.5) .DELTA.E. There is a need for a coating that can be applied to a pressing surface without changing the appearance of the finish on the pressing surface. There is a need for a pressing surface that is not scratched when used in pressing laminate coated with alumina particles of greater than 10 microns and preferably greater than 25 microns. There is a particular need for a pressing surface that is not scratched when used in pressing high gloss laminate with an ASTM 2457 60 degree angle gloss of greater than 70, when the surface of the laminate is coated with 25-60 micron alumina particles.